1. Technical Field of the Invention
The present invention relates to an in-cylinder injection type internal combustion engine adapted to inject fuel directly into a combustion chamber and to cause the injected fuel to be spark-ignited for stratified combustion, and more particularly, to a system for raising the exhaust-gas temperature to thereby enable immediate activation of an exhaust-gas purification device of an in-cylinder injection type internal combustion engine when the engine continuously effects a lean combustion operation.
2. Description of the Related Art
For spark-ignition type automotive internal combustion engines, various in-cylinder injection type gasoline engines have been proposed. These types of engines inject fuel directly into a combustion chamber, unlike conventional intake-manifold injection type engines, in which fuel is injected into an intake manifold and transferred to a combustion chamber. An in-cylinder injection type engine is typically arranged to inject fuel from a fuel injection valve into a cavity formed in the top of a piston of the engine, to locally generate an air-fuel mixture having an air-fuel ratio close to the stoichiometric air-fuel ratio around an ignition plug at ignition timing. This enables a proper combustion of a lean air-fuel mixture, whose average air-fuel ratio observed in the entirety of the combustion chamber is lean, reduce the emission of harmful exhaust-gas components, and improve fuel consumption. However, if the engine performs such a lean-combustion operation through the entire operating region, a deficient engine output may occur in some operating condition. To obviate this, the in-cylinder injection type engine is arranged to switch the injection mode between a compression-stroke injection mode and an intake-stroke injection mode depending upon engine operating conditions.
When the engine is in a low-load operating region, the compression-stroke injection mode is selected, in which fuel is injected mainly during compression stroke. In this mode, most of the fuel, injected toward the cavity of the piston during compression stroke, stays in the cavity because of the action of a tumble flow of intake air sucked into the combustion chamber during intake stroke. Therefore, even if such a small amount of fuel, as to make the average air-fuel ratio large, is injected (compression-lean mode), an air-fuel mixture having an air-fuel ratio close to the stoichiometric air-fuel ratio is formed in the cavity around a spark plug, at ignition timing, at which the piston approaches the spark plug. Hence, the, inflammation of the air-fuel mixture by a spark may become possible. This permits a large amount of intake air to be supplied into the cylinder in the compression-stroke injection mode, so that pumping loss is decreased and fuel consumption is greatly improved.
When the engine is in a medium- or high-load operating region, fuel is injected mainly during intake stroke, so as to form a mixture with a uniform air-fuel ratio in the combustion chamber. In this case, a large amount of fuel can be burnt without causing any misfire due to the presence of an overrich mixture around the spark plug, whereby the engine output required at the time of acceleration or during high speed running of a vehicle can be ensured.
At the time of cold start of engine, or during a low-load engine operation at a low ambient air temperature, an in-cylinder injection type internal combustion engine may take much time to activate a catalyst of an exhaust-gas purification device disposed in the exhaust passage of the engine. When the engine is operated in the compression-lean mode where a large amount of intake air is supplied into a cylinder, the flow rate of exhaust gas is high, and hence the exhaust-gas temperature tends to become low. Accordingly, the catalyst may fail to maintain its activated temperature, when the engine is operated in the compression-lean mode even if the catalyst has once reached the activation temperature. To eliminate these problems, various methods are proposed for raising the exhaust-gas temperature to effect a rapid activation of catalyst.
For example, an in-cylinder injection type internal combustion engine proposed in JP-A-4-183922 operates a fuel injection valve during compression stroke of the engine to inject a main fuel into a combustion chamber, and actuates a spark plug to ignite the main fuel. Then, the fuel injection valve is operated again during expansion stroke or during an early stage of exhaust stroke in which the intake valve is kept closed, to thereby inject an additional fuel into the combustion chamber, and the spark plug is actuated again to ignite the additional fuel. However, the proposed system requires a complicated ignition-control logic and cannot produce sufficient energy for the second ignition.
In this regard, JP-A-8-100638 proposes a method for permitting an additional fuel to be burnt without utilizing spark ignition. In the proposed method, a main fuel is injected during compression stroke of an engine, a spark plug is actuated to ignite the main fuel, and an additional fuel is injected during expansion stroke. A flame produced by the inflammation of the main fuel initiated upon spark-operation of the spark plug propagates to the additional fuel and causes the same to be burnt. By this method, the additional fuel can be burnt without the need of re-actuating the spark plug, and the combustion of the additional fuel raises the exhaust-gas temperature, to thereby shorten a time required for activation of the catalyst.
However, according to the proposed method, the additional fuel must be injected during that time period in which the additional fuel can be enflamed by the flame which propagates during the main combustion. Actually, in the proposed method, the injection timing of the additional fuel is set to a value ranging from, e.g., 10.degree. to 80.degree. ATDC in terms of crank angle. However, if the additional fuel is injected during an early stage of expansion stroke like this, part of the thermal energy produced at the time of combustion of the additional fuel is wasted for the work of expansion, so that an intended rise of the exhaust-gas temperature may not be sufficiently achieved. Furthermore, an amount of additional fuel must be increased in order to sufficiently raise the exhaust-gas temperature, causing the fuel consumption to be further worsened.